High pressure experiments suggest liquid iron could flow through the mantle.

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Our planet’s interior is complex and has many layers. There are many unsolved mysteries about the formation and structure of these layers, but new research is providing some clues about how Earth’s internal structure may have evolved.

If you were to take a journey to the center of the Earth, you would find that most material there is made of just three elements, at least until you get to around 3000 km below the surface. These elements—oxygen, silicon, and magnesium (plus a little bit of iron)—make up more than 90% of Earth’s “ceramic” mantle. Electrically and thermally insulating, the minerals of the mantle are the stony part of the planet.

But as you go deeper, things suddenly change. About midway to the center, you cross a boundary from the stony mantle into the metallic core, initially liquid in its upper stretches, and then solid right in the center of the Earth. The chemistry changes, too, with almost all of the core being composed of iron.

The boundary between the metallic core and the rocky mantle is a place of extremes. In physical characteristics, Earth’s metallic liquid outer core is as different from the rocky mantle as the seas are from the ocean floor. One might imagine an inverted world that has storms and currents of flowing red-hot metal in the molten outer core. It is this flow of metal in the core that gives Earth its magnetic field, protects us from the solar storms that constantly bombard us, and allows life to thrive.

How did such distinct layers of material end up next to each other? In a paper published in the Nature Geoscience, a group of scientists led by Wendy Mao of Stanford University have shown how metallic iron may be squeezed out of rocky silicates at depths of around 1000 km beneath the crust.

Enlarge/ The percolation of iron deep in the Earth provides a multi-stage route to forming a core, early in the planet’s history.

Nature Geoscience

Experiments on mixtures of silicate minerals and iron cooked up in the lab show that iron sits in tiny isolated lumps within the rock, remaining trapped and pinned at the junctions between the mineral grains. This observation has led to the view that iron only segregates in the early stages of planetary formation, when the upper part of the silicate mantle is fully molten. It is thought that droplets of iron rained down through the upper mantle and pooled at its base, then sank as large “diapir” driven by gravity. These fell through the deeper solid mantle to eventually form a core.

Mao’s work suggests that this model needs revising. The team used intense X-rays to probe samples held at extreme pressure and temperature squeezed between the tips of diamond crystals. They found that when pressure increases deep into the mantle, iron liquid begins to wet the surfaces of the silicate mineral grains. This means that threads of molten iron can join up and begin to flow in rivulets through the solid mantle, a process called percolation. More importantly, this process can occur even when the mantle is not hot enough to form a magma ocean.

“In order for percolation to be efficient, the molten iron needs to be able to form continuous channels through the solid,” Mao explained. “Scientists had said this theory wasn’t possible, but now we’re saying, under certain conditions that we know exist in the planet, it could happen. So this brings back another possibility for how the core might have formed.”

Commenting on the results, Geoffrey Bromiley of the University of Edinburgh said, “This new data suggests that we cannot assume that core formation is a simple, single-stage event. Core formation was a complex, multi-stage process which must have had an equally complex influence on the subsequent chemistry of the Earth.”

Mao’s data raises important questions about how we start the formation of cores in planets. The prevailing idea in earth sciences is that studying the cores of meteorites and asteroids may help reveal insights about our own planet. But, Bromiley said, “their deep percolation model implies that early core formation can only be initiated in large planets. As a result, the chemistry of the Earth may have been ‘reset’ by core formation in a markedly different way from smaller planets and asteroids.”

He added, “The challenge now lies in finding a way to model the numerous processes of core formation to understand their timing and subsequent influence on the chemistry of not just the Earth, but also the other rocky bodies of the inner solar system.”

Bromiley and his colleagues are now investigating whether other factors might influence structure formation, like the deformation that asteroids and other bodies might have experienced on their chaotic pathways through the early Solar System. His work is adding other interesting questions. “We are increasingly observing metallic cores in bodies much smaller than the Earth. What process might have aided core formation in bodies which were never large enough to permit percolation of core forming melts at great depths?”

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The Conversation is an independent source of news and views, sourced from the academic and research community. Our team of editors work with these experts to share their knowledge with the wider public. Our aim is to allow for better understanding of current affairs and complex issues, and hopefully improve the quality of public discourse on them.

im interested in the "ceramic" mantle bit - what do they mean by that? the innate(crystalline etc) properties? or is it more like metallicity for stars? (which as i understand it doesnt follow the usual usage of the word 'metal')...

I think there is more to the movement of metal within the planet than is commonly thought. Can someone explain why metal forms veins, for instance? As far as I understand it, orthodoxy states that the metal that is here was here since the beginning and came from the very materials that formed the solar system. But it has always seemed to me that it is more likely that metal is actually created in the center of the planet. Is there a theory of why metal "veins"?

EDIT: Specifically, if gold is essentially non-reactive, I don't understand why it wouldn't be randomly distributed.

But it has always seemed to me that it is more likely that metal is actually created in the center of the planet.

Alchemy?!

Metals are chemical elements (as opposed to chemical compounds). To "create" a chemical element, that is to convert one chemical element into another, you need a nuclear reaction.

Fission or fusion, or radioactive decay of heavier radioisotope into lighter ones. Those are your choices.

Center of the Earth is a rather extreme environment when compared to the surface, but it isn't even CLOSE to hot enough for fusion. Natural fission has stopped billions of years ago.

That leaves us with naturally occurring radioisotopes decaying into lighter stable elements. Those processes are well known. Most of lead found today came from decay of Uranium, for example. There are no decay chains possible to give us so much iron.

Iron was created by nuclear fusion. In a place where stable sustained nuclear fusion is possible: inside stars.

Sun is a 2nd generation star. Somewhere nearby there was a 1st generation star. It spent billions of years fusing hydrogen into helium, then when it ran out of hydrogen it fused ever heavier elements until it exploded. The remnant nebula, now rich with heavy elements, collapsed under gravity to create our Sun and the accretion disk which formed the planets.

Sun is a 2nd generation star. Somewhere nearby there was a 1st generation star. It spent billions of years fusing hydrogen into helium, then when it ran out of hydrogen it fused ever heavier elements until it exploded. The remnant nebula, now rich with heavy elements, collapsed under gravity to create our Sun and the accretion disk which formed the planets.

I've watched many shows on the universe that explain the same thing you have. They never explain though where the hydrogen for the Sun came from if the first start fused all of it up. Was it outside of the star held back by the solar wind or in an outer layer of the start, some other place? If it was there when the first star formed, why wasn't it IN the first start. If it wasn't how did it get to our neighborhood?

But it has always seemed to me that it is more likely that metal is actually created in the center of the planet.

Alchemy?!

Metals are chemical elements (as opposed to chemical compounds). To "create" a chemical element, that is to convert one chemical element into another, you need a nuclear reaction.

Fission or fusion, or radioactive decay of heavier radioisotope into lighter ones. Those are your choices.

Center of the Earth is a rather extreme environment when compared to the surface, but it isn't even CLOSE to hot enough for fusion. Natural fission has stopped billions of years ago.

That leaves us with naturally occurring radioisotopes decaying into lighter stable elements. Those processes are well known. Most of lead found today came from decay of Uranium, for example. There are no decay chains possible to give us so much iron.

Iron was created by nuclear fusion. In a place where stable sustained nuclear fusion is possible: inside stars.

Sun is a 2nd generation star. Somewhere nearby there was a 1st generation star. It spent billions of years fusing hydrogen into helium, then when it ran out of hydrogen it fused ever heavier elements until it exploded. The remnant nebula, now rich with heavy elements, collapsed under gravity to create our Sun and the accretion disk which formed the planets.

You caught my alchemy reference!

How do we know how hot it is in the center of the earth? Is that measurement reliable? Could it be radically adjusted or do we have truly reliable methods to constrain the possible values?

What? If it were hot enough you would be sitting on the surface of a star. Are you?

Physicists know how to calculate pressures inside a body collapsed under its own gravity, and from there temperatures. We know what sort of temperatures are required for nuclear fusion. Hint: we've done it, not that it matters because we know how to calculate it, too. And then different scientists called geologists and seismologists and volcanologists, sometimes blow stuff up, or wait for an earthquake to occur, and use their instruments to measure those values directly. Surprise they match what the physicists were saying all along.

I've watched many shows on the universe that explain the same thing you have. They never explain though where the hydrogen for the Sun came from if the first start fused all of it up. Was it outside of the star held back by the solar wind or in an outer layer of the start, some other place? If it was there when the first star formed, why wasn't it IN the first start. If it wasn't how did it get to our neighborhood?

Good questions! Here are some attempted explanations. The basic points are that the first star did not use up all of its hydrogen or even close, most of the hydrogen that was in the vicinity of the star did not end up in the star, and that there is still plenty of hydrogen (most abundant element in the universe by far) that has not yet been part of a star at all.

You may understand that large stars explode in a supernova when they run out of fusion fuel. That's not quite right. It's when it runs out of fuel in the core, where temperatures are high enough for fusion to take place. What happens is that as heavier elements are fused, they naturally accumulate in the core. Assuming the star is big enough to fuse those elements, it then creates heavier ones which also accumulate and you get a nice stratified layers of elements from heaviest to lightest. Which means hydrogen is pushed out of the core and is no longer available for fusion. When the star reaches iron, fusion is now energy negative, and the chain reaction will stop once there's too much iron in the core (which happens in a matter of days once it begins). This causes the collapse and subsequent explosion.

All of the hydrogen that was not in the star's core undergoing fusion -- the majority of the star's mass -- is released along with all the heavy elements (and the even heavier elements created by the force of the supernova itself).

Large stars in particular are very inefficient consumers of hydrogen and return most of it back out into space to maybe one day form a star again.

Smaller stars like our sun that won't explode still return most of their hydrogen back to space for recycling. Fusion in our sun will stop at carbon, at which point the energy of fusion will be sufficient to blow off the outer layers of our star and leave just the core as a white dwarf.

And then there's all the hydrogen still out there, floating around in enormous masses but not of sufficient density to collapse and form stars. One way to turn such a could of gas into an open star cluster is for the material ejected from a supernova to hit it, compressing it and starting the process of collapsing into stars, which simultaneously seeding it with heavy elements.

Once those stars ignite, they will as you correctly surmised begin blowing the rest of the gas away due to their stellar winds and even light output. Particularly the large stars that form all the heavy elements will quickly cut off further accumulation.

So there you go, there's plenty of hydrogen for a second, third, and many many future generations of stars. Hope that helps.

Adding on to Wymhole's excellent comment I'd add that while the smallest type of stars, red dwarfs probably will use most of their hydrogen up because they have a convection cycle running from the surface down into the core that because of their trillion year life cycles what they release on death isn't a factor in what goes into current generations of stars.

I've watched many shows on the universe that explain the same thing you have. They never explain though where the hydrogen for the Sun came from if the first start fused all of it up. Was it outside of the star held back by the solar wind or in an outer layer of the start, some other place? If it was there when the first star formed, why wasn't it IN the first start. If it wasn't how did it get to our neighborhood?

Good questions! Here are some attempted explanations. The basic points are that the first star did not use up all of its hydrogen or even close, most of the hydrogen that was in the vicinity of the star did not end up in the star, and that there is still plenty of hydrogen (most abundant element in the universe by far) that has not yet been part of a star at all.

You may understand that large stars explode in a supernova when they run out of fusion fuel. That's not quite right. It's when it runs out of fuel in the core, where temperatures are high enough for fusion to take place. What happens is that as heavier elements are fused, they naturally accumulate in the core. Assuming the star is big enough to fuse those elements, it then creates heavier ones which also accumulate and you get a nice stratified layers of elements from heaviest to lightest. Which means hydrogen is pushed out of the core and is no longer available for fusion. When the star reaches iron, fusion is now energy negative, and the chain reaction will stop once there's too much iron in the core (which happens in a matter of days once it begins). This causes the collapse and subsequent explosion.

All of the hydrogen that was not in the star's core undergoing fusion -- the majority of the star's mass -- is released along with all the heavy elements (and the even heavier elements created by the force of the supernova itself).

Large stars in particular are very inefficient consumers of hydrogen and return most of it back out into space to maybe one day form a star again.

Smaller stars like our sun that won't explode still return most of their hydrogen back to space for recycling. Fusion in our sun will stop at carbon, at which point the energy of fusion will be sufficient to blow off the outer layers of our star and leave just the core as a white dwarf.

And then there's all the hydrogen still out there, floating around in enormous masses but not of sufficient density to collapse and form stars. One way to turn such a could of gas into an open star cluster is for the material ejected from a supernova to hit it, compressing it and starting the process of collapsing into stars, which simultaneously seeding it with heavy elements.

Once those stars ignite, they will as you correctly surmised begin blowing the rest of the gas away due to their stellar winds and even light output. Particularly the large stars that form all the heavy elements will quickly cut off further accumulation.

So there you go, there's plenty of hydrogen for a second, third, and many many future generations of stars. Hope that helps.

Sun is a 2nd generation star. Somewhere nearby there was a 1st generation star. It spent billions of years fusing hydrogen into helium, then when it ran out of hydrogen it fused ever heavier elements until it exploded. The remnant nebula, now rich with heavy elements, collapsed under gravity to create our Sun and the accretion disk which formed the planets.

I've watched many shows on the universe that explain the same thing you have. They never explain though where the hydrogen for the Sun came from if the first start fused all of it up. Was it outside of the star held back by the solar wind or in an outer layer of the start, some other place? If it was there when the first star formed, why wasn't it IN the first start. If it wasn't how did it get to our neighborhood?

I think you may be taking the first poster a little bit to literally. Initially (after the first really crazy bits) the universe was almost entirely Hydrogen and Helium, with a (relatively) wee bit of Lithium. Eventually the first stars formed, were crazy large and short lived, and burned themselves up. In that process they started forming the first of the heavier elements. That continues to this day.

So really it was this process repeated over billions of years that 'seeded' our region of space with the heavier elements (beyond H, He and Li) needed to form the Earth, the other rocky planets, moons, comets, asteroids, etc. The H and He in our star was already here, thanks to the Big Bang (in theory) and it eventually became intermixed with the elements from exploded stars somewhere in the general neighborhood.

Sun is a 2nd generation star. Somewhere nearby there was a 1st generation star. It spent billions of years fusing hydrogen into helium, then when it ran out of hydrogen it fused ever heavier elements until it exploded. The remnant nebula, now rich with heavy elements, collapsed under gravity to create our Sun and the accretion disk which formed the planets.

I've watched many shows on the universe that explain the same thing you have. They never explain though where the hydrogen for the Sun came from if the first start fused all of it up. Was it outside of the star held back by the solar wind or in an outer layer of the start, some other place? If it was there when the first star formed, why wasn't it IN the first start. If it wasn't how did it get to our neighborhood?

All hydrogen is 'primordial', it was formed in the big bang. In fact to a pretty good degree of precision the conventional products of the big bang were protons, electrons, and photons, light and hydrogen. There was a bit of helium in there too from a bit of primordial fusion, and maybe a VERY small trace of lithium (element 3).

In the more immediate sense stars form from neutral hydrogen clouds, which are themselves formed out of clouds of ionized hydrogen in some not-well-understood process. All of this hydrogen is primordial, it simply never before was incorporated into a star. Under the influence of gravity and probably with the help of shock waves from supernova these clouds collapse to form stars. The supernova as well as the outflow from T-Tauri stars and other types of stars which blow off material supply heavier elements. Remember, even in the Sun hydrogen is the vast majority of the mass. Earth is basically the distilled iron and rock from a MUCH larger mass of primordial material, most of which was hydrogen that the early Sun's heat drove off (and the Earth was not massive enough to retain). Jupiter is massive because it DID retain this material out in the cold outer system, and once it grew big enough to hold tight to hydrogen it probably grew very fast to its current size. It probably started out only modestly bigger than Earth.

How do we know how hot it is in the center of the earth? Is that measurement reliable? Could it be radically adjusted or do we have truly reliable methods to constrain the possible values?

Probably not. We know the mass of the Earth, and its volume quite precisely. We thus know its density, and using seismometry we know the density profile as well. This doesn't leave a lot of room for big variations. The temperature gradients can only be so large in any case or else the heat would drive extreme convection, and we have a pretty good idea of the speed of convection at the surface, so that's another check on things. I am not sure what the error bars are exactly on the temperature of the core, but they aren't 1000's of kelvins. A 100 K one way or another isn't likely to make a vast difference overall.

I think there is more to the movement of metal within the planet than is commonly thought. Can someone explain why metal forms veins, for instance? As far as I understand it, orthodoxy states that the metal that is here was here since the beginning and came from the very materials that formed the solar system. But it has always seemed to me that it is more likely that metal is actually created in the center of the planet. Is there a theory of why metal "veins"?

EDIT: Specifically, if gold is essentially non-reactive, I don't understand why it wouldn't be randomly distributed.

I don't think the conceptual problem is too hard. Imagine a liquid like magma containing a variety of minerals, and cool it. At different temperatures, as it cools, different substances will no longer dissolve and will crystalize out forming a layer of mainly the substance that has reached the point of no longer dissolving; so we get successive layers of different minerals, which then get bent and messed up by subsequent geologic processes.

Perhaps fissionable elements should be mentioned. Iron doesn’t melt on its own.

My point/question related to the temperature of the Earth’s core. Without the heat from fission, my understanding was that the Earth’s core would cool and no longer maintain the magnetosphere. Such cooling of Mar’s smaller core supposedly allowed the solar wind to strip its atmosphere. Thus it’s a bit disconcerting to learn that all the fissionable material at the Earth’s core has been consumer and we’re coasting thermally. But early on the heat from fission supposedly did melt the iron in the core. Molten iron collecting at the core is rather straight forward once the heavier elements collected and went critical.

I don't think the conceptual problem is too hard. Imagine a liquid like magma containing a variety of minerals, and cool it. At different temperatures, as it cools, different substances will no longer dissolve and will crystalize out forming a layer of mainly the substance that has reached the point of no longer dissolving; so we get successive layers of different minerals, which then get bent and messed up by subsequent geologic processes.

I'll add to that.

Elements with similar chemical properties will be harder to separate, that's why you tend to find iridium as a trace metal in platinum, and platinum as a trace metal in nickel. A good rule of thumb is that where you find a given element, you will find other elements from the same column of the periodic table, along with one or two elements to either side.

Naturally that's not the full story, as sometimes compounds of nearby elements can have vastly different properties (for example solubility of nitrates vs carbonates in water)and result in further separation under the right circumstances.

Perhaps fissionable elements should be mentioned. Iron doesn’t melt on its own.

My point/question related to the temperature of the Earth’s core. Without the heat from fission, my understanding was that the Earth’s core would cool and no longer maintain the magnetosphere. Such cooling of Mar’s smaller core supposedly allowed the solar wind to strip its atmosphere. Thus it’s a bit disconcerting to learn that all the fissionable material at the Earth’s core has been consumer and we’re coasting thermally. But early on the heat from fission supposedly did melt the iron in the core. Molten iron collecting at the core is rather straight forward once the heavier elements collected and went critical.

Some of that may be taking place, but remember Earth also has a lot more mass to cool down, and a greater distance from the core to the surface. It would take about twice as long for the core to cool, even without additional heat input.

Some heating would also be caused by the action of tidal forces. My gut tells me it's not very much, but I'd rather see if someone with a better handle on the physics verify that.

Thanks for the great answer! It's really helped things fall into place. in the core was the key part I was missing. You even answered another question I've had from the shows: how long does it take for the star to go supernova once it starts making iron? They've all made it sound like it happens almost instantly, but that always seemed "off" to me though because iron is relatively abundant. Though I guess compared to a star's lifespan, days might as well be an instant.

I've watched many shows on the universe that explain the same thing you have. They never explain though where the hydrogen for the Sun came from if the first start fused all of it up. Was it outside of the star held back by the solar wind or in an outer layer of the start, some other place? If it was there when the first star formed, why wasn't it IN the first start. If it wasn't how did it get to our neighborhood?

Good questions! Here are some attempted explanations. The basic points are that the first star did not use up all of its hydrogen or even close, most of the hydrogen that was in the vicinity of the star did not end up in the star, and that there is still plenty of hydrogen (most abundant element in the universe by far) that has not yet been part of a star at all.

You may understand that large stars explode in a supernova when they run out of fusion fuel. That's not quite right. It's when it runs out of fuel in the core, where temperatures are high enough for fusion to take place. What happens is that as heavier elements are fused, they naturally accumulate in the core. Assuming the star is big enough to fuse those elements, it then creates heavier ones which also accumulate and you get a nice stratified layers of elements from heaviest to lightest. Which means hydrogen is pushed out of the core and is no longer available for fusion. When the star reaches iron, fusion is now energy negative, and the chain reaction will stop once there's too much iron in the core (which happens in a matter of days once it begins). This causes the collapse and subsequent explosion.

All of the hydrogen that was not in the star's core undergoing fusion -- the majority of the star's mass -- is released along with all the heavy elements (and the even heavier elements created by the force of the supernova itself).

Large stars in particular are very inefficient consumers of hydrogen and return most of it back out into space to maybe one day form a star again.

Smaller stars like our sun that won't explode still return most of their hydrogen back to space for recycling. Fusion in our sun will stop at carbon, at which point the energy of fusion will be sufficient to blow off the outer layers of our star and leave just the core as a white dwarf.

And then there's all the hydrogen still out there, floating around in enormous masses but not of sufficient density to collapse and form stars. One way to turn such a could of gas into an open star cluster is for the material ejected from a supernova to hit it, compressing it and starting the process of collapsing into stars, which simultaneously seeding it with heavy elements.

Once those stars ignite, they will as you correctly surmised begin blowing the rest of the gas away due to their stellar winds and even light output. Particularly the large stars that form all the heavy elements will quickly cut off further accumulation.

So there you go, there's plenty of hydrogen for a second, third, and many many future generations of stars. Hope that helps.

I think there is more to the movement of metal within the planet than is commonly thought. Can someone explain why metal forms veins, for instance? As far as I understand it, orthodoxy states that the metal that is here was here since the beginning and came from the very materials that formed the solar system. But it has always seemed to me that it is more likely that metal is actually created in the center of the planet. Is there a theory of why metal "veins"?

EDIT: Specifically, if gold is essentially non-reactive, I don't understand why it wouldn't be randomly distributed.

this has been a great discussion, but I felt the replies missed one of your questions that I will take a stab at...

So the 1st Generation star exploded leaving enough hydrogen for the Sun and heavy elements for the planets.. during this Accretion period, the debris from the supernova collect into larger and larger bodies along with many small bodies... asteroids and comets.

Now I don't think there is consensus on if a supernova blows out big chunks of Iron core and uranium the size of houses.. so I think your even distribution question has merit for the accretion period. But how they end up as veins in the Earth is well understood. Asteroid, plate tectonics, and time.

Picture this.... the newly formed Earth finally cools so the surface is no longer covered in Magma. There is still a large amount of debris floating in space but many of those bodies have also coalesced into asteroids and comets. Now, the majority of asteroids we have observed were ~90% iron, so say one of these big chunks of metal fall to the fresh surface of the earth. Sure you get an explosion and a crater but its more like a baseball landing in the mud rather than a Hiroshima bomb. The asteroid (now meteorite) stays mostly in tact.

So you have a big metal ball embedded in the rocky ground. Now we wait... a LONG time.Geologic time is slow. rock don't move very fast. but they do move. if you could speed up time you could see that the Earth's crust moves around like taffy being stretched. all the pushing and pulling and pressure generates heat. the metallic ball of our asteroid doesn't stand a chance and goes with the flow of the rest of the taffy.. getting stretched and pulled along with the rest of the rock around it.

a billion years later some dwarf with pickaxe comes by and finds the vein of metal that was once a space rock stretched all the way through the misty mountain.

Mao’s data raises important questions about how we start the formation of cores in planets.

So now Ars is writing articles for mouse-like hyper-intelligent pan-dimensional beings? The readership here never ceases to amaze me.

I'm sure they mean: "how we start the formation" [in our models].

The Article wrote:

“We are increasingly observing metallic cores in bodies much smaller than the Earth. What process might have aided core formation in bodies which were never large enough to permit percolation of core forming melts at great depths?” The Conversation

Could it be that the asteroids with metallic cores originally came from larger planets which had previously broke apart? The prevailing theory of solar formation states that there were originally dozens (even hundreds) of planetoids, all which vied for the stable orbits around the sun. There were a lot of collisions and mergers, and a lot of debris ejected into space. (Heck, even today's Earth-Moon system is the product of two primordial planets impacting.)

Most debris eventually collected into winning 8 planets, but some of it stayed in asteroids.

Perhaps fissionable elements should be mentioned. Iron doesn’t melt on its own.

My point/question related to the temperature of the Earth’s core. Without the heat from fission, my understanding was that the Earth’s core would cool and no longer maintain the magnetosphere. Such cooling of Mar’s smaller core supposedly allowed the solar wind to strip its atmosphere. Thus it’s a bit disconcerting to learn that all the fissionable material at the Earth’s core has been consumer and we’re coasting thermally. But early on the heat from fission supposedly did melt the iron in the core. Molten iron collecting at the core is rather straight forward once the heavier elements collected and went critical.

The core of the Earth is a giant thermal mass at a high temperature covered with a blanket of pretty good insulating material, the mantle, that is 3000km deep. While the mantle DOES convect it does so on very long geological timescales (overturn rates measured in 100's of megayears). So the interior of the Earth cools VERY slowly. It probably has cooled a considerable amount in 4.5 gigayears but it is still quite hot.

Remember, Mars is MUCH smaller than the Earth. It has something like 1/10th the mass of the Earth, thus it could cool far more quickly. Limited plate tectonics probably did happen on Mars early on but it stopped long ago and was never the prevalent system it is on Earth. Even Venus, at .815 Earth mass, seems to have run out of steam in that department, though the speculation is it is more related to lack of water in the mantle than cooling. Still, we don't know exactly, the Earth may be close to the minimum size required for a planet to have plate tectonics, or it may be a very rare phenomenon that requires extremely special circumstances. Likewise the magnetic field produced by the core. The thing is we don't actually know that said magnetic field actually IS necessary for life. In fact AFAIK it is not considered necessary for the Earth to retain an atmosphere. This notion is quite prevalent but I distinctly remember reading a couple of actual studies which refuted that.

I think there is more to the movement of metal within the planet than is commonly thought. Can someone explain why metal forms veins, for instance? As far as I understand it, orthodoxy states that the metal that is here was here since the beginning and came from the very materials that formed the solar system. But it has always seemed to me that it is more likely that metal is actually created in the center of the planet. Is there a theory of why metal "veins"?

EDIT: Specifically, if gold is essentially non-reactive, I don't understand why it wouldn't be randomly distributed.

this has been a great discussion, but I felt the replies missed one of your questions that I will take a stab at...

So the 1st Generation star exploded leaving enough hydrogen for the Sun and heavy elements for the planets.. during this Accretion period, the debris from the supernova collect into larger and larger bodies along with many small bodies... asteroids and comets.

Now I don't think there is consensus on if a supernova blows out big chunks of Iron core and uranium the size of houses.. so I think your even distribution question has merit for the accretion period. But how they end up as veins in the Earth is well understood. Asteroid, plate tectonics, and time.

Supernova are unimaginably violent events. The energy release is so vast, often on the order of the entire lifetime output of the Sun in a split second, that nothing physical can possibly survive. If the Earth was hit just by the neutrino flux from a supernova that was where the Sun is now just the million jillionth of a fraction of one percent of those neutrinos that would be absorbed by the Earth would be enough to utterly dissociate it into its component atoms, ionize them all, and accelerate the resulting plasma to a significant fraction of the speed of light. There are no chunks of iron, or any chunks of anything, resulting from this process. As the debris cools it forms a very very fine dust of silica, iron, carbon, and other common fission products. These then mix with interstellar hydrogen and if said hydrogen happens to collapse into another star, then they are carried along for the ride. The galaxy is filled with quite a few dust clouds that are remnants of past star formation/destruction.

Quote:

Picture this.... the newly formed Earth finally cools so the surface is no longer covered in Magma. There is still a large amount of debris floating in space but many of those bodies have also coalesced into asteroids and comets. Now, the majority of asteroids we have observed were ~90% iron, so say one of these big chunks of metal fall to the fresh surface of the earth. Sure you get an explosion and a crater but its more like a baseball landing in the mud rather than a Hiroshima bomb. The asteroid (now meteorite) stays mostly in tact.

Possibly. A LOT of energy is released in these impacts, which at least tends to melt/vaporize the impactor. I think you would get a giant 'splash' and there would be a bunch of iron left at the impact site, and some spread around. Manicouagan I believe is actually mined for some of that iron.

Quote:

So you have a big metal ball embedded in the rocky ground. Now we wait... a LONG time.Geologic time is slow. rock don't move very fast. but they do move. if you could speed up time you could see that the Earth's crust moves around like taffy being stretched. all the pushing and pulling and pressure generates heat. the metallic ball of our asteroid doesn't stand a chance and goes with the flow of the rest of the taffy.. getting stretched and pulled along with the rest of the rock around it.

a billion years later some dwarf with pickaxe comes by and finds the vein of metal that was once a space rock stretched all the way through the misty mountain.

Well, I think basically that is what happens. However the vast majority of iron ore we have on Earth is primordial, not from later impacts. Even though most of the iron sank to the core it was so common an element that the fraction left bound into silicates and other forms on the surface and mantle is still very significant. Billions of years ago when the Earth had a reducing atmosphere much of this iron was in solution in the oceans. As bacteria and plants poured oxygen into the atmosphere via photosynthesis this iron oxidized and sank to the bottom of bodies of water as basically rust, forming banded iron formations. Eventually all the iron was oxidized and then oxygen had noplace else to go, so it piled up in the atmosphere, probably causing all sorts of havoc, massive ice ages, etc for possibly as long as 2 billion years before levels stabilized at something approaching modern ones in the Cambrian.